The present invention relates generally to optical (laser) pointing acquisition and tracking (PAT) systems and methods, and more particularly, to improved acquisition through the use of hybrid RF/optical tracking systems and methods.
Conventional optical pointing acquisition and tracking systems are very slow to acquire PAT on the received signal. Furthermore, optical communication systems typically do not have backup systems that are operative in the case of optical path interruption due to obscuration or excessive beam jitter.
It is an objective of the present invention to provide for improved optical beam pointing, acquisition and tracking by means of hybrid RF/optical pointing acquisition and tracking systems and methods.
To meet the above and other objectives, the present invention provides for a hybrid optical (laser) and millimeter wave beam acquisition and tracking system for an airborne optical atmospheric or space communications link along with an associated beam acquisition and tracking method. The primary function of the hybrid optical and millimeter wave transmission system is transmission of information over an optical medium. The millimeter wave medium is provided to assist in the pointing, and acquisition of tracking of the optical beam, and secondly, to serve as a backup communications medium in the event that the optical path is interrupted due to obscuration or excessive optical beam jitter.
An exemplary beam acquisition and tracking system comprises a host platform having an inertial navigation system and a global positioning system for generating geolocation data, an optical receiver, a radio frequency (RF) receiver, and a common optical/RF aperture. An optical/RF interface is coupled to the host platform by way of a mechanical gimbal that supports a beamsplitter, an optical gimbal and optical feed for coupling optical beam energy from the beamsplitter to the optical receiver, an RF feed for coupling RF energy from the beamsplitter to the RF receiver, and a central processing unit comprising an optical track error processor for acquiring the optical beam by systematically searching for a focused light spot as it appears in a focal plane of the optical receiver, an RF track error processor for outputting gimbal angles of the mechanical gimbal that are derived from the aperture of the RF receiver and that are referenced to the inertial navigation system, and a search and track processor coupled to the optical track error processor and RF track error processor that generates an optical gimbal control signal for the optical gimbal, and generates a mechanical gimbal control signal for the mechanical gimbal for tracking the optical and RF beams.
An exemplary beam acquisition and tracking method comprises the following steps. A host platform is provided that contains an inertial navigation system and a global positioning system for generating geolocation data, an optical receive r, a radio frequency (RF) receiver, a common optical/RF aperture, and an optical/RF interface coupled to the host platform by way of a mechanical gimbal that comprises a beamsplitter, an optical gimbal and optical feed for coupling optical beam energy from the beamsplitter to the optical receiver, and an RF feed for coupling RF energy from the beamsplitter to the RF receiver.
The ephemeris angles of the mechanical gimbal are initially estimated through knowledge of the host platform""s attitude, its geolocation and the geolocation of the cooperating laser transmitter (or cooperating terminal that emits the beam that is to be acquired). These estimates are facilitated by the inertial navigation system and the global positioning system. An optical ambiguity region associated with the RF beam pointing is systematically searched until it is acquired. The focal plane of the optical receiver is systematically searched until the optical beam is acquired. (Fast steering mirrors or other methods are used for search and track of the optical signals). Control signals for the optical gimbal and a mechanical gimbal control system are generated for tracking the optical and RF beams.
The RF and optical elements of the hybrid system share a common aperture on the host platform. It is estimated that the incremental cost of the millimeter wave components are a small fraction of the optical system. The improvement in functionality, reduced optical beam acquisition time and backup for optical path outage, justifies the increased cost/complexity.
The present invention provides for reduced beam acquisition time in an airborne laser communications system. The present invention employs a systematic and exhaustive search of an ambiguity window requiring multiple complex hardware or very long acquisition time. The present invention reduces acquisition time and provides an RF backup carrier during optical outages. The present invention does not require a large radome since the millimeter wave antenna shares a common window (aperture) with the optical aperture.